The long range goal of this research program is to understand processes of sef-assembly and regulation in multisubunit protein systems. The immediate aim is to elucidate mechanisms of protein-protein interaction and protein-ligand interactions in specific systems by: (a) characterizing the interactions in thermodynamic (and kinetic) terms, and (b) relating these properties to specific molecular structures of the interacting molecules. The research program has two major components: (1) Development of improved methods for analyzing the distributions of molecular species in reversibly interacting systems. The use of molecular sieve techniques and the further development of these methods as analytical tools for determining stoichiometries and equilibrium constants are basic approaches of the program. The use of microcalorimetry to obtain directly the thermodynamic parameters, once stoichiometries have been defined will also continue to be emphasized. (2) Studies of ligand-mediated subunit interaction. The use of subunit dissociation is being developed as a means of probing energy changes within intersubunit contact regions of quaternary complexes, as they are altered via ligand binding during their cycles of functional transition. Resolution of the linkage between subunit interaction and ligand binding will be pursued in human hemoglobin, lamprey hemoglobin and in enzyme systems such as glyceraldehyde-3-phosphate dehydrogenases. The emphasis in studies proposed here is on detailed thermodynamic characterization of several systems for which structures are known.